Unmanned Aerial Vehicles (UAVs) are unpiloted aircraft that are either controlled remotely or are autonomously flown based on pre-programmed flight plans. UAVs are also commonly categorized based on their design and performance specifications that span the range from miniature low altitude to large High Altitude Long Endurance (HALE) vehicles. HALE UAVs could provide improved service over existing systems in a large number of civil applications, ranging from border patrol and coastal surveillance, monitoring of natural disasters, meteorology and cartography to highly flexible telecommunication relay stations. For example, platforms capable to remain airborne for weeks to months at altitudes of about 10-25 km provide advantages over satellite systems in terms of reduced costs, increased flexibility and higher precision.
The UAV technology is taking an increasingly important place in our society for civilian and military applications. The required endurance is in the range of a few hours in the case of law enforcement, border surveillance, forest fire fighting or power line inspection. However, other applications at high altitudes, such as communication platform for mobile devices, weather research and forecast, environmental monitoring, would require remaining airborne for days, weeks, months or even years. It is possible to reach these goals using electric solar powered platforms. Photovoltaic (PV) cells and modules may be used to collect the solar energy during the day, a part of which may be used directly for maintaining flight and onboard operations with the remainder being stored for the night time.
The use of sunlight as a source of energy for aircraft has many compelling advantages. Solar energy entails zero marginal cost, weight, and emissions per hour of flight. Sunlight provides a maximum of about 1000 W/m2 at sea level, but reaches a more abundant 1400 W/m2 at high altitudes unobstructed by cloud cover. With advances in efficient and lightweight materials for collector, storage, and wing structures, solar aircraft can aspire to sustain flight at high altitudes for days, weeks, even years.
One approach to building a solar airplane is to cover the upper wing surfaces of a plane with photovoltaic cells. This configuration works best when the sun is directly overhead, but it suffers a loss of power proportional to cosine of the angle between the normal of the wing surface and the sun direction. In 1998 the DLR Institute of Flight Systems built the “Solitair” prototype with solar panels that tilt along a single axis to orient towards the sun. This approach works best when the plane can fly in the direction perpendicular to the solar azimuth, but otherwise also suffers a cosine power loss. Such panels also create turbulence, aerodynamic instability and drag. In 1999-2003, AeroVironment and NASA developed the “Helios” UAV prototype. “Helios” wing is segmented into several solar-cell-covered sections connected by hinged joints. The joints allow tilting some of the sections towards the sun, but do not significantly compensate for the cosine power reduction.
A different approach to a “Solar Thermal Aircraft” is disclosed in U.S. Pat. No. 7,270,295 of C. L. Bennett. As shown in FIG. 7 of the above referenced patent, the solar collector is a reflective parabolic trough (110) mounted to rotate freely around its focal axis in an optically transparent section of the aircraft body. A solar tracker aligns the reflective trough with the sun, to concentrate sunlight onto a heat pipe (120) along the focal axis, thereby heating a fluid which transfers solar energy to a heat engine (140) that propels the aircraft.
In contrast to other solar aircraft that propose solar collectors within the airframe of a plane, Bennett's aircraft body and wing pod designs are aerodynamically inefficient. Bennett proposes the optically transparent portion of the fuselage skin to be a strong ultraviolet-resistant polymer film, such as DuPont TEDLAR®, which has excellent transparency, tensile strength, and low weight. However the fixed drag-to-lift characteristics of Bennett's fuselage design will increase the propulsion power needed to remain on station, even at night when the solar collectors are idle.
The entire prior art solar aircraft referenced above suffer a cosine law reduction of power when the sun's direction is not parallel to the normal of the solar panels. The Bennett trough, “Solitair” panels, and other tilting tail wing designs all provide single-tilt compensation only for the banking or roll angle of sun relative to the ideal overhead position. For example, none of these designs can harvest solar power when the aircraft is flying directly towards or away from the sun. Tilting panels and wings should not provide aerodynamic lift, because resulting forces would compromise aerodynamic stability and may even damage the aircraft. Thus, such tilting collectors are essentially pure drag elements, and like the Bennett fuselage will increase propulsion power needed to remain on station, including at night time when the solar panels are idle.